Improve the way to define and use date-format.

_layouts/tag.html

@@ -7,8 +7,6 @@ layout: page
 # MIT Licensed
 ---
 
-{% include date-format.html %}
-
 <div id="page-tag">
   <h1 class="pl-lg-2">
     <i class="fa fa-tag fa-fw text-muted"></i>
@@ -20,7 +18,7 @@ layout: page
     <li class="d-flex justify-content-between pl-md-3 pr-md-3">
       <a href="{{ post.url | absolute_url }}">{{ post.title }}</a>
       <span class="dash flex-grow-1"></span>
-      <span class="text-muted small">{{ post.date | date: POST_DATE }}</span>
+      <span class="text-muted small">{{ post.date | date: site.data.date_format.post }}</span>
     </li>
     {% endfor %}
   </ul>